Resistive memory is one of the emerging memory technologies that may replace dynamic random access memory (DRAM) as the main memory in computers. Resistive memory in general refers to any technology that uses varying cell resistance to store information. One type of resistive memory is metal-oxide resistive random access memory (ReRAM).
A ReRAM cell has a metal-oxide layer sandwiched between two metal electrodes. A low resistance state (LRS or ON-state) and a high resistance state (HRS or OFF-state) are used to represent the logical “1” and ‘0’ respectively or vice versa. In order to switch a ReRAM cell, an external voltage with certain polarity, magnitude, and duration is applied to the metal oxide. The switching of LRS-to-HRS is called a reset operation and the switching of HRS-to-LRS is called a set operation.
A ReRAM-based main memory may be fabricated in a crossbar with a high cell density. In the crossbar, all ReRAM cells are interconnected to each other with nonlinear selectors instead of access transistors. The cells are directly sandwiched between wordlines and bitlines. To set a cell, the selected wordline and bitline are set to voltages V and 0 respectively. Conversely to reset a cell, the selected wordline and bitline are set to voltages 0 and V respectively.
A single cell in a crossbar may be accessed by applying the proper potential across the wordline and bitline connected to the cell. Ideally when a wordline and a bitline are activated, the entire current should flow through the cell that lies at their crossing point. However other cells in the selected wordline and bitline also see partial voltage across them. The half-selected cells in the selected wordline and bitline leak current through them due to partial write voltage across them, which is commonly referred to as a sneak current. In order to reduce the sneak current, all the other wordlines and bitlines that are not selected are half biased at V/2. This limits the voltage drop on the half-selected cells to V/2 and the voltage drop on the unselected cells to 0.
Use of the same reference numbers in different figures indicates similar or identical elements.